The present invention relates to a process for the purpose of removing the binder phase from the surface of cemented carbide inserts before applying coatings on said surface and the resulting inserts.
Coated cemented carbide inserts for machining of metals in the metal cutting industry have been commercially available for many years. Such inserts are commonly made of a metal carbide, normally WC, generally with addition of carbides of other metals such as Nb, Ti, Ta, etc., and a metallic binder phase of cobalt. By depositing onto said inserts a thin coating of a wear resistant material such as TiC, TiN, Al.sub.2 O.sub.3, etc., separately or in combination, it has been possible to increase the wear resistance while maintaining essentially the same toughness. A still further improvement in properties has been obtained by subjecting the inserts to a binder phase enrichment in the surface below the coating, so-called cobalt gradient. Binder phase enrichment can be accomplished, for instance, by sintering in vacuum with nitride addition as is disclosed in U.S. Patent RE 34,180 (a reissue of U.S. Pat. No. 4,610,931), or by controlled cooling as disclosed in U.S. Pat. No. 4,911,989. Such inserts, however, often also have a thin layer of binder phase on their surface and sometimes even with a layer of graphite thereon.
The two latter layers have a negative effect on the CVD- or PVD-deposition process, which results in deposited coatings with inferior properties and insufficient adherence. These surface layers must therefore be removed before carrying out the deposition process.
Blasting of cemented carbide inserts is a common method in the art for cleaning the surface of the inserts prior to coating. The blasting is generally performed wet or dry with particles with a size of about 150 mesh. It is, of course, possible to remove said cobalt- and possible graphite-layers by such conventional blasting. However, the method is difficult to control with regard to the blasting depth, especially close to the cutting edge. In this area, the cobalt gradient zone is very easily removed which leads to an increased scatter in the properties of the final product--the coated insert. In addition, conventional blasting results in damages to the carbide grains and uneven removal of the cobalt layer which can give inferior adherence of the coating.
Chemical or electrolytic methods are alternatives for mechanical methods as disclosed in , e.g., U.S. Pat. No. 4,282,289, U.S. Pat. No. 4,911,989, JP 88-060279, JP 88-060280 and JP 88-053269. There is one serious drawback with these methods, namely, that they are incapable of only removing the cobalt layer. They also result in deep etching, particularly in areas close to the edge. As a result, an undesired porosity between the coating and the substrate is obtained in one area of the insert at the same time as the cobalt layer may partly remain in other areas of the insert. A solution to this problem is found in Swedish patent application SE 9101469-6, where is disclosed an improved method of electrolytic etching in a mixture containing concentrated sulphuric acid, H.sub.2 SO.sub.4, and concentrated phosphoric acid, H.sub.3 PO.sub.4. This method gives the desired effect of cleanly and effectively removing the surface layer of binder metal and any graphite from the surface of a cemented insert. An excellent surface with little cobalt and no damage to the carbide grains is obtained. However, using the method under production conditions is not completely simple. Concentrated acids have to be handled and explosive or health damaging gases and vapors may develop which have to be removed.